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Assessing Weekly GPS Position Errors and Load Corrections

Quantify the error budget for weekly dNEU GPS positions and use IGS solutions for a global set of 706 stations. Includes results from a subset of 119 close station pairs to isolate electronic/thermal noise component. Assess draconitic errors and spatial distribution as well as consistency of crustal loading signals derived from models and GPS.

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Assessing Weekly GPS Position Errors and Load Corrections

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  1. Quantify error budget for weekly dNEU GPS positions • use IGS solutions for a global set of 706 stations • Includeresultsfromsubset of 119 close station pairs • isolate electronic/thermal noise component • Assessdraconiticerrorstoo • spatial distribution & station vs. satellite source Consistency of Crustal Loading SignalsDerived from Models & GPS:Inferences for GPS Positioning Errors Jim Ray, NOAA/National Geodetic Survey Xavier Collilieux & Paul Rebischung, IGN/LAREG Tonie van Dam, University of Luxembourg Zuheir Altamimi, IGN/LAREG AGU Fall 2011, Session G51B-06, San Francisco, 9 December 2011

  2. Input Data Sets to Compare • GPS station position time series from IGS reprocessing • analysis consistent with IERS 2010 Conventions (more or less) • combined results from up to 11 Analysis Centers • 706 globally distributed stations, each with >100 weeks • data from 1998.00 to 2011.29 • Helmert alignment (no scale) w.r.t. cumulative solution uses a well-distributed subnetwork to minimize aliasing of local load signals • care taken to find position/velocity discontinuities • Mass load displacement time series for the same stations • 6-hr NCEP atmosphere • 12-hr ECCO non-tidal ocean • monthly LDAS surface ice/water, cubic detrended to remove model drift • all computed in CF frame • sum is linearly detrended & averaged to middle of each GPS week • data from 1998.0 to 2011.0 • Study dN, dE, dU non-linear residuals (1998.0 – 2011.0) • bias errors not considered here ! 02

  3. (GPS – Load) Comparison Statistics • Load corrections are effective to reduce WRMS & annual amps • for most stations, esp for dU – but less for dN & even less for dE 03

  4. (GPS – Load) Comparison Statistics • Load corrections are effective to reduce WRMS & annual amp • But most residual variation remains, esp in dN & dE 04

  5. A Generalized Model of Position WRMS • WRMS2 = WRMSo2 + (Ai * AnnAmpi)2 + WRMSi2 • WRMSo2 = globally averaged error floor, including: • basement electronic & thermal noise (presumably white) • a priori modeling errors (tides & basic geophysics) • other large-scale analysis errors (e.g., orbits) • AnnAmpi = mean annual amplitude • Ai = 1 / sqrt(2) = 0.7071 or Ai2 = 0.5 → for stationary sinusoid • Ai2 > 0.5 → for non-stationary seasonal variations • includes loads + all other annual effects (e.g., technique errors) • WRMSi2 = local site-specific errors (non-annual part only): • multipath & monument noise • antenna mis-calibration • thermal expansion of antenna installation & bedrock • tropomis-modeling + geography-dependent orbit errors • non-annual loads (or residual load model errors, if corrected) • inter-AC analysis & station usage differences + RF realization 05

  6. Model of Position WRMS – Global Error Floor WRMSoerror floor 06

  7. Model of Position WRMS – Annual Components add variable amounts of annual variation: AnnAmpi / sqrt(2) 07

  8. Model of Position WRMS – Local Errors add variable amounts of local variations: WRMSi 08

  9. IGS Results – dN With/Without Loads A2 : 0.5 0.6 • Load corrections have no impact on noise floor assessment • local site & non-load errors overwhelmingly dominate 09

  10. IGS Results – dE With/Without Loads A2 : 0.5 0.6 • Load corrections have no impact on noise floor assessment • local site & non-load errors overwhelmingly dominate 10

  11. IGS Results – dU With/Without Loads A2 : 0.5 0.6 • Load corrections move results much closer to noise floor • but local site & non-load errors still dominate 11

  12. IGS Results – dU With/Without Loads A2 : 0.5 0.6 GLPS LAGO • “Best” 2 stations in dU (WRMS = 2.2 mm) are: • LAGO (S. coast, Portugal) & GLPS (island, Pacific Ocean) 12

  13. IGS Results – dU With/Without Loads LAGO (coast) GLPS (island) dN & dE scatters (~1 mm) are not the “best” . . . possibly related to weaknesses of IB assumption or load models dU scatters have same WRMS but different characters 13

  14. Error Floors Inferred from Nearby Station Pairs • Single-station deconvolution from observed pair differences by 1/sqrt(2) 14

  15. Error Floors Inferred from Nearby Station Pairs • Good agreement of short-baseline & global WRMSo estimates implies that orbit errors are not significant part of error floor 15

  16. Decomposition of Weekly GPS Position Errors • Infer site WRMSi2 using: • WRMSo2 + (A * AnnAmpi)2 + WRMSi2 = WRMS2 • where A2 = 0.6 • Noise floorWRMSo & local site errorsWRMSidominate over loads • esp for N & E components • Non-load GPS annualerrors are as large as annualloadsignals • unless load models missing ~half of total signal 16

  17. Distribution of 4th GPS Draconitic in dU • Draconitic year = 351.2 d • or frequency = 1.04 cpy • fit (1.0 + 2.0 + 3.12 + 4.16) cpy • phases clockwise from N • w.r.t. 2000.0 • plot significant 4th harmonics • Strong spatial correlations • very striking in most regions • also seen in dN distribution • esp coherent in Europe • even for tiny dE signals • Implies orbit-related source • at least for most of effect • smaller local contributions • also likely based on close • pairs 17

  18. Distribution of 4th GPS Draconitic in dU GPS orbit inclination = 55° GPS orbit inclination = 55° longitudes of GPS orbit node Right Ascensions 18

  19. 4th GPS Draconitic in Europe dN dE dU 19

  20. Summary & Conclusions • Load corrections reduce WRMS & annual amps for most stations • but most residual variation remains, esp for dN & dE • implies load models are inaccurate &/or other sources of scatter/error • Station weekly scatter can be decomposed into 3 categories: • dNdEdU • global average floor WRMSo (mm) 0.65 0.7 2.2 • median annual amp WRMS (mm) 0.7 0.6 2.8 • (for A2 = 0.6; about half due to loads) • median local site WRMSi (mm) 1.0 1.1 2.9 • total WRMS (mm) 1.4 1.45 4.6 • Harmonics of GPS draconitic period are pervasive • strong spatial correlations imply a major orbit-related source (see Griffiths & Ray, G54A-01, this afternoon) • but significant draconitics in close pair differences imply smaller local contributions likely too • 1.04 cpy signal must contribute to observed annual variations but magnitude is unknown • Major non-load technique improvements still badly needed! 20

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